1. Field of the Invention
The present invention relates to an image coding system, and particularly a compression coding system of a still picture or a motion picture.
2. Description of the Related Art
With regard to compression coding of a still picture or a motion picture, there is a demand of coding one frame within a predetermined number of coding bits with higher efficiency.
To fulfill such a demand, the following methods have been taken conventionally. As the first method, given that B is the total number of bits for one frame and N is the number of blocks to be encoded within one frame, the number of bits per block is defined by B/N and a coding process is taken while executing bit adjustment in such a way that the number of bits of each block approaches B/N (see "Scene Adaptive Coder," by W. H. Chen and W. K. Pratt, IEEE Trans. COM-32, No. 3 1984). The second method is to subject each block to DCT (discrete cosine transformation), transfer only the first one of the transformation factors to be transferred, as counted from a low region, for every transfer block, then only the second transformation factor as counted from the low region and so forth, and stop this operation when a predetermined number of bits are reached.
Neither method, however, has a local property within a frame taken into consideration, thus providing a reproduced image with different coding errors for different blocks. This would increase block distortion or cause blur or the like at the edges of a fine picture image portion, thus deteriorating the image quality even from the point of subjective evaluation.